1. Field of the Invention
The present invention relates to a communication device, and more particularly to a communication device which controls packet transmission.
2. Description of the Related Art
Recent years have seen increasing amounts of packet traffic because of the expanding use of the Internet. In today's Internet Protocol (IP) networks, the packets carry various types of information, from ordinary computer data to delay-sensitive realtime voice and video streams. To address the requirements for packet transport with smaller delays, a new traffic engineering protocol called “label switching” has been proposed. Label is a short fixed-length value that is attached to packets at the ingress node to specify a path to a particular destination. Without using ordinary layer-3 (network layer) routers, label-switched networks transport such labeled packets over a layer-2 path that is previously defined for each destination. That is, the label switching mechanism enables fast packet delivery by eliminating upper-layer routing procedures. This new technology, the Multiprotocol Label Switching (MPLS) protocol, is currently under standardization by the Internet Engineering Task Force (IETF).
FIG. 7 shows a conventional way of transporting IP packets with label switching techniques. In the network shown in FIG. 7, each network node 400 to 404 has a kind of hybrid configuration of a label switch part 400a to 404a and an ordinary router part 400b to 404b. The label switches 400a to 404a are designated by their unique identifiers L400 to L404, respectively. Notice that there are two routes RA and RB between two end nodes 400 and 403, the latter route being shorter than the former in terms of the number of intermediary nodes and links.
The basic function of the label switches 400a to 404a is to forward packets from source to destination by using label information. To this end, they receive an IP packet having a label and determine whether the identifier contained in the label field matches with the identifier of its own. If they agree with each other, the label switches 400a to 404a pass the received packets to their local router part 400b to 404b, removing the label from each packet. If not, the packets are forwarded to the next-hop link on their pre-established label-switched path.
Suppose, for example, that a label L403 is attached to an IP packet at the ingress node 400. This packet is transported through the label switches in the intermediary nodes, or transit nodes, until it finally reaches the egress node 403, where the packet is directed from the label switch 403a to the router 403b. In this way, label-switched networks deliver IP packets to their destinations without using layer-3 routing mechanisms, thus making faster transport of packets possible.
The above-described conventional way of label switching, however, lacks flexibility in its route selection functions. More specifically, the conventional label switching provides only a fixed set of label-switched paths being defined and stored in a routing table at each source end. The problem is that such packet routes registered in a routing table are not guaranteed to be the shortest or optimal ones, which sometimes causes inefficiency in the packet transport. In the example of FIG. 7, the second route RB is shorter than the first route RA. However, in the case where the routing table happens to include the route RA as the best route to take, the IP packets addressed to the node 403's domain will follow the route RA, spending more time than when the other route RB is taken.